1. Field of the Invention
This invention is related in general to conductive materials used to manufacture clear conductive surfaces. In particular, it pertains to a combined sputtering/vapor-deposition process for manufacturing conductive flakes suitable for conductive inks and coatings.
2. Description of the Related Art
Conductive surfaces are used in many modern applications. For example, touch screens and electrodes for electroluminescent displays incorporate a conductive layer made of conductive metallic or metal-oxide (ceramic) particles dispersed in a clear flexible binder applied to a clear flexible substrate. The conductivity of the layer results from the contact among the individual conductive particles in the dispersion.
Typically, these conductive surfaces are manufactured by dispersing a metallic or metal-oxide powder in a binder and applying the mixture as a coating over a clear substrate. Indium tin oxide (ITO) and indium zinc oxide (IZO) are clear conductive metal oxides most often used for these applications; gold, silver and copper are used to produce opaque conductive layers. The powders are manufactured chemically by precipitation from a solution or mechanically by grinding solid nuggets. The resulting powder particles, normally in the order of nanometers in nominal diameter, tend to be highly reflective, rather than optically transmissive, even though the material in thin-film form may be transparent. This is because the light scattering produced by reflection of dispersed random-shape particles greatly reduces the transparency of the bulk material, just as in the case of pulverized glass particles. Therefore, the application of a conductive powder to a clear substrate such as plastic or glass, while producing a conductive surface, tends to yield a translucent but not perfectly clear layer even when ITO or IZO is used.
It has been known that the metal-oxide compounds commonly used in the manufacture of conductive layers, most commonly ITO and IZO, remain transparent in flake form because of the high aspect ratio associated with the flake form (i.e., the ratio of the nominal diameter of the two-dimensional surface to the thickness of the flake). Accordingly, the use of metal-oxide particles in flake form to manufacture clear conductive surfaces has been known to be desirable for some time. Unfortunately, the only known methods for producing a conductive flake coating involve the processes of sputtering or reactive electron-beam evaporation of the conductive compound with direct deposition onto the clear substrate, which can only be achieved by placing the substrate into the process chamber. This approach is impractical, or even impossible, for large devices that may not fit in conventional vacuum chambers and for surfaces with a complicated geometry that cannot be reached uniformly by sputtering or vapor deposition.
Therefore, the common practice has been to produce translucent conductive layers by coating a clear substrate with inks that contain a metal oxide powder, such as ITO or IZO, dispersed in a binder. This technique has the advantage of allowing the application of the conductive layer to a substrate of any geometry and size, but also has the attendant disadvantage of yielding a relatively opaque product with relatively low conductivity. The present invention is directed at providing a solution to these problems by manufacturing inks and coatings that contain conductive flakes, rather than powders.